#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
string s;
cin >> n >> s;
auto get_val = [&](char c) {
if(c == 'R') return 0;
else if(c == 'G') return 1;
else return 2;
};
vector<int> pos[3];
for(int i = 0; i < n; ++i) {
pos[get_val(s[i])].emplace_back(i);
}
int cnt[3], mx = -1;
for(int i = 0; i < 3; ++i) {
cnt[i] = pos[i].size();
mx = max(mx, cnt[i]);
}
if(2 * mx > n + 1) {
cout << -1;
return 0;
}
vector pref(3, vector<int>(n + 1, 0));
for(int i = 0; i < n; ++i) {
for(int c = 0; c < 3; ++c) {
pref[c][i + 1] = pref[c][i];
}
pref[get_val(s[i])][i + 1]++;
}
const ll inf = (1ll << 60);
vector dp(cnt[0] + 1, vector(cnt[1] + 1, array<ll, 4>{})), ndp(dp);
auto set_val = [&](auto& a) {
for(int i = 0; i <= cnt[0]; ++i) {
for(int j = 0; j <= cnt[1]; ++j) {
for(int z = 0; z < 4; ++z) {
a[i][j][z] = inf;
}
}
}
};
auto umin = [&](ll& a, ll b) {
if(a > b) a = b;
};
auto get_cost = [&](int col, int id, int r, int g, int y) {
int p = pos[col][id];
int used[3] = {r, g, y};
ll C = 0;
for(int i = 0; i < 3; ++i) {
C += max(0, pref[i][p] - used[i]);
}
return C;
};
set_val(dp);
dp[0][0][3] = 0;
for(int used = 0; used < n; ++used) {
set_val(ndp);
for(int r = 0; r <= cnt[0]; ++r) {
for(int g = 0; g <= cnt[1]; ++g) {
int y = used - r - g;
if(y < 0 || y > cnt[2]) continue;
for(int z = 0; z < 4; ++z) {
ll cur = dp[r][g][z];
if(cur == inf) continue;
if(r < cnt[0] && z != 0)
umin(ndp[r + 1][g][0], cur + get_cost(0, r, r, g, y));
if(g < cnt[1] && z != 1)
umin(ndp[r][g + 1][1], cur + get_cost(1, g, r, g, y));
if(y < cnt[2] && z != 2)
umin(ndp[r][g][2], cur + get_cost(2, y, r, g, y));
}
}
}
dp.swap(ndp);
}
ll ans = inf;
for(int i = 0; i < 3; ++i)
umin(ans, dp[cnt[0]][cnt[1]][i]);
cout << ans;
return 0;
}
